Technique for Correcting Hotspots in Mask Patterns and Write Patterns

ABSTRACT

Embodiments of a method for determining a mask pattern to be used on a photo-mask in a lithography process are described. This method may be performed by a computer system. During operation, this computer system receives at least a portion of a first mask pattern including first regions that violate pre-determined rules associated with the photo-mask. Next, the computer system determines a second mask pattern based on at least the portion of the first mask pattern, where the second mask pattern includes second regions that are estimated to comply with the pre-determined rules. Note that the second regions correspond to the first regions, and the second mask pattern is determined using a different technique than that used to determine the first mask pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and techniques for determiningwrite patterns for maskless-lithography processes that use write devicesand mask patterns for lithography processes that use photo-masks.

2. Related Art

Lithography processing represents an essential technology formanufacturing Integrated Circuits (IC) and Micro Electro-MechanicalSystems (MEMS). Lithographic techniques are used to define patterns,geometries, features, shapes, etc. onto an integrated-circuit die,semiconductor wafer, or chips, where the patterns are typically definedby a set of contours, lines, boundaries, edges, curves, etc., whichgenerally surround, enclose, and/or define the boundary of the variousregions which constitute the patterns.

One existing lithographic technique is photolithography, in which imagesdefined by photo-masks are printed onto the integrated-circuit die orthe semiconductor wafers. Furthermore, another existing lithographictechnique is maskless lithography, in which a write device directlyprints a write pattern onto the integrated-circuit die or thesemiconductor wafers, thereby eliminating the need for photo-masks.Unfortunately, it is increasingly difficult to determine the writepatterns, or to design and manufacture photo-masks.

In particular, demand for increased density of features on theintegrated-circuit die and semiconductor wafers has resulted in thedesign of circuits with decreasing minimum dimensions. These trends havesignificantly increased the complexity of the computations necessary todetermine the write patterns and/or the mask patterns (to which thephoto-masks correspond), with a commensurate impact on computation time,processing requirements, and expense.

Furthermore, due to the wave nature of light, as dimensions approachsizes comparable to the wavelength of the light used in thephotolithography processes, the resulting wafer patterns deviate fromthe corresponding photo-mask patterns and are accompanied by unwanteddistortions and artifacts. Existing techniques (such as opticalproximity correction or OPC, and resolution enhancement technologies orRET) are used to pre-distort the mask patterns to improve resolutionand/or to improve a process window (i.e., a range of process conditionsthat result in acceptable yield) in a photolithography process.

While these techniques may ensure that the wafer pattern is printed moreaccurately, determining the pre-distorted mask patterns is increasinglydifficult, thereby exacerbating the computational complexity and theassociated problems. For example, computing the pre-distorted maskpatterns may be complicated by the presence of multiple potentialsolutions (such as local minima) in a higher-dimensional solution space.Moreover, while many of these solutions may produce similar waferpatterns, it may be easier to manufacture photo-masks corresponding tosome of the solutions than others. Consequently, identifying a suitablepre-distorted mask pattern may be time-consuming and expensive.

Moreover, many of the mask patterns and/or write patterns determinedusing existing techniques include one or more regions (which aresometimes referred to as hotspots) that violate pre-determined rulesassociated with the lithographic and/or photolithographic processes. Asthe density of features increases, it is increasingly difficult tocorrect the hotspots (so that these regions comply with thepre-determined rules) using existing techniques.

Hence, what is needed is a method and an apparatus that facilitatesdetermination of write patterns and mask patterns without theabove-described problems.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method fordetermining a mask pattern to be used on a photo-mask in a lithographyprocess, which may be performed by a computer system. During operation,this computer system receives at least a portion of a first mask patternincluding first regions that violate pre-determined rules associatedwith the photo-mask. Next, the computer system determines a second maskpattern based on at least the portion of the first mask pattern, wherethe second mask pattern includes second regions that are estimated tocomply with the pre-determined rules. Note that the second regionscorrespond to the first regions, and the second mask pattern isdetermined using a different technique than that used to determine thefirst mask pattern.

In some embodiments, the computer system further receives the first maskpattern.

In some embodiments, the computer system further analyzes the first maskpattern using the predetermined rules to identify the first regions andextracts at least the portion of the first mask pattern. Note that theanalysis of the first mask pattern may involve verification. Moreover,the verification may be image-based.

In some embodiments, the identification of the first regions involvesidentifying locations associated with violations of the pre-determinedrules, and the extraction of at least the portion of the first maskpattern involves calculating the first regions based on one or moregeometric relationships for shapes surrounding the locations. Forexample, the one or more geometric relationships may include overlappingshapes or adjacent shapes, such as polygons, that contact each otheralong an edge. Moreover, in some embodiments the extraction of at leastthe portion of the first mask pattern involves aggregating thecalculated first regions.

In some embodiments, the receiving involves accessing at least theportion of the first mask pattern in a computer-readable memory.

In some embodiments, the computer system further applies an OpticalProximity Correction (OPC) to at least the portion of the first maskpattern prior to the determination of the second mask pattern.

In some embodiments, the computer system further merges the second maskpattern with a remainder of the first mask pattern, where the remainderof the first mask pattern excludes the first regions.

In some embodiments, the first regions include hotspots.

In some embodiments, the pre-determined rules include manufacturingrules associated with the photo-mask.

In some embodiments, at least the portion of the first mask patternfurther includes third regions, where a given third region surrounds agiven first region. Note that the third regions may be unchanged duringthe determination of the second mask pattern. Moreover, in someembodiments a second portion of the third regions are changed during thedetermination of the second mask pattern, and this second portion of thethird regions is a transition region between the second regions and thefirst portion of the second regions.

In some embodiments, an optical characteristic of the second portion ofthe third regions is approximately unchanged or is improved relative tothe optical characteristic of the second portion of the third regions inat least the portion of the first mask pattern. For example, the opticalcharacteristic may include intensity, contrast, depth of focus, and/or amask error enhancement factor.

In some embodiments, the first mask pattern includes multiple subsetsthat are to be processed by a group of processors, where the portion ofthe first mask pattern includes those multiple subsets that areproximate to the first regions. Note that in some embodiments at leastsome of the subsets are processed in parallel.

In some embodiments, the computer system further analyzes the secondmask pattern using the predetermined rules to confirm that the secondregions comply with the pre-determined rules. Note that the analysis ofthe second mask pattern may involve verification.

In some embodiments, the determination of the second mask patterninvolves an inverse lithographic calculation that includes a projectionof a target pattern at an image plane in the photolithographic processto an object plane in the photolithographic process.

In some embodiments, the determination of the second mask patterninvolves image-based or pixel-based correction.

In some embodiments, at least the portion of the first mask pattern isincluded in a file that is compatible with a GDSII format.

Another embodiment provides the computer system configured to executeinstructions corresponding to at least some of the above-describedoperations.

Another embodiment provides a computer-program product for use inconjunction with the computer system, which includes instructionscorresponding to at least some of the above-described operations.

Another embodiment provides a semiconductor wafer. This semiconductorwafer is produced in the photolithography process that uses thephoto-mask.

Another embodiment provides the photo-mask.

Another embodiment provides a data file stored in a computer-readablememory that includes information corresponding to the mask pattern.

Another embodiment provides another method in which at least some of theabove-described operations are used for generating a write pattern(instead of a mask pattern) to be used in a maskless-lithographicprocess.

Another embodiment provides another computer-program product for use inconjunction with the computer system, which includes instructionscorresponding to at least some of the above-described operations forgenerating the write pattern.

Another embodiment provides another semiconductor wafer. Thissemiconductor wafer is produced in the maskless-lithography process.

Another embodiment provides another data file stored in acomputer-readable memory that includes information corresponding to thewrite pattern.

One embodiment of the present invention provides a third method fordetermining a mask pattern to be used on a photo-mask in a lithographyprocess, which may be performed by another computer system. Duringoperation, this other computer system receives a file including a firstmask pattern and additional information associated with regions in thefirst mask pattern that violate pre-determined rules associated with thephoto-mask. Next, the other computer system determines a second maskpattern based on the first mask pattern, wherein the second mask patternis estimated to comply with the predetermined rules.

In some embodiments, the regions include hotspots.

In some embodiments, the information is included in a single-layer inthe file. Moreover, the file may be compatible with a GDSII format.

In some embodiments, the regions include polygons, where a given polygonsurrounds a portion of the first mask pattern that violates thepre-determined rules.

In some embodiments, the pre-determined rules include manufacturingrules associated with the photo-mask.

In some embodiments, the regions are determined during verification.

In some embodiments, the other computer system further performsverification on the second mask pattern to confirm that the second maskpattern complies with the pre-determined rules. Note that theverification may be image-based.

In some embodiments, the determination of the second mask patterninvolves image-based and/or pixel-based correction.

In some embodiments, the determination of the second mask patterninvolves an inverse lithographic calculation that includes a projectionof a target pattern at an image plane in the photolithographic processto an object plane in the photolithographic process.

In some embodiments, the second mask pattern is determined using adifferent technique than that used to determine the first mask pattern.

Another embodiment provides the other computer system configured toexecute instructions corresponding to at least some of theabove-described operations.

Another embodiment provides a third computer-program product for use inconjunction with the other computer system, which includes instructionscorresponding to at least some of the above-described operations.

Another embodiment provides a third semiconductor wafer. This thirdsemiconductor wafer is produced in the photolithography process thatuses the photo-mask.

Another embodiment provides the photo-mask.

Another embodiment provides a third data file stored in acomputer-readable memory that includes information corresponding to themask pattern.

Another embodiment provides a fourth method in which at least some ofthe above-described operations are used for generating a write pattern(instead of a mask pattern) to be used in a maskless-lithographicprocess.

Another embodiment provides a fourth computer-program product for use inconjunction with the other computer system, which includes instructionscorresponding to at least some of the above-described operations forgenerating the write pattern.

Another embodiment provides a fourth semiconductor wafer. Thissemiconductor wafer is produced in the maskless-lithography process.

Another embodiment provides a fourth data file stored in acomputer-readable memory that includes information corresponding to thewrite pattern.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1A is a block diagram illustrating determination of a mask patternand/or a write pattern in accordance with an embodiment of the presentinvention.

FIG. 1B is a block diagram illustrating determination of a mask patternand/or a write pattern in accordance with an embodiment of the presentinvention.

FIG. 2A is a block diagram illustrating a mask pattern in accordancewith an embodiment of the present invention.

FIG. 2B is a block diagram illustrating a mask pattern in accordancewith an embodiment of the present invention.

FIG. 2C is a block diagram illustrating a mask pattern in accordancewith an embodiment of the present invention.

FIG. 3 is a flowchart illustrating a process for generating the maskpattern and/or the write pattern in accordance with an embodiment of thepresent invention.

FIG. 4 is a flowchart illustrating a process for generating the maskpattern and/or the write pattern in accordance with an embodiment of thepresent invention.

FIG. 5 is a block diagram illustrating a computer system to generate themask pattern and/or the write pattern in accordance with an embodimentof the present invention.

FIG. 6 is a block diagram illustrating a data structure in accordancewith an embodiment of the present invention.

FIG. 7 is a block diagram illustrating a data structure in accordancewith an embodiment of the present invention.

Note that like reference numerals refer to corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

Embodiments of a computer system, a method, a computer program product(i.e., software), and a data structure or a file for use with thecomputer system are described. These systems, processes, and/or datastructures may be used to determine or generate mask patterns that areused to produce photo-masks, which in turn are used to produceintegrated-circuit die and/or semiconductor wafers (henceforth referredto as semiconductor wafers) during a photolithographic process.Furthermore, in some embodiments the systems, processes, and/or datastructures are used to determine or generate write patterns that areused to produce the semiconductor wafers during a maskless-lithographicprocess (such as a process that uses a direct write device). Inparticular, the photo-mask may be used to project an aerial image orprint a pattern (henceforth referred to as the printed pattern)corresponding to a target pattern in a design (such as a design layer inan integrated circuit) on to a semiconductor wafer. Alternatively, awrite device (such as a laser writer or an e-beam writer) may use thewrite pattern to write a pattern on to the semiconductor wafer. In thediscussion that follows, determination or generating of a mask patternis used as an illustrative example.

In some embodiments, a file containing at least the regions in a maskpattern that violate pre-determined rules (such as design and/ormanufacturing rules) associated with the photo-mask is received. Theseregions may include so-called hotspots in the mask pattern. Next,another mask pattern may be determined based on at least the regionsusing a different technique than that used to determine the maskpattern. This other mask pattern may include additional regions, whichcorrespond to the regions, and which comply with the pre-determinedrules.

In some embodiments, a remainder of the mask pattern, which excludes theregions, is merged with the other mask pattern. Moreover, in someembodiments, the other mask pattern is analyzed (for example, duringverification) to confirm that the additional regions comply with thepredetermined rules.

In some embodiments, at least the regions are identified in the maskpattern (for example, during verification) and are extracted to producethe file. However, in some embodiments the file includes the maskpattern as well as information associated with the regions. Thisinformation may be included in a single layer in the file. Note that thefile may be compatible with a GDSII format.

In an example embodiment, the mask pattern is at least in partdetermined using a technique that includes Optical Proximity Correctionor OPC. Moreover, the determination of the other mask pattern mayinvolves an inverse lithographic calculation that includes a projectionof a target pattern at an image plane in the photolithographic processto an object plane in the photolithographic process.

We now describe embodiments of a system or technique for determining orgenerating mask patterns and/or write patterns. FIG. 1A presents a blockdiagram 100 illustrating an embodiment of determining a mask patternand/or a write pattern. During the determining, a pattern generator 112determines mask or write pattern 114-1 based on a target pattern 110,such as the target pattern to be produced on a semiconductor die duringa photolithographic or lithographic process. Next verifier 116-1 mayanalyze mask or write pattern 114-1 to determine if it complies withpre-determined rules, such as those associated with manufacturing of aphoto-mask to be used in the photo-lithographic process.

If one or more regions in the mask or write pattern 114-1 violate one ormore of the pre-determined rules, hot-spot extractor 118 may extractthese regions and output hotspots 120. Then, hotspot-repair generator122 may modify the regions such that they are estimated to comply withthe pre-determined rules.

In some embodiments, a merge 124 component combines the modified regionswith the mask or write pattern 114-1 (or a remainder of the mask orwrite pattern 114-1 that excludes the regions) and outputs a new mask orwrite pattern 114-2. Moreover, in some embodiments an optional verifier116-2 confirms that the modified regions comply with the pre-determinedrules.

In some embodiments, at least some of the functions illustrated in blockdiagram 100 are performed by different parties. For example, a firstparty may generate the mask or write pattern 114-1. Then, a second partymay perform verification and/or extraction of hotspots 120. Thesehotspots and the mask or write pattern 114-1 (or the remainder of themask or write pattern 114-1 that excludes the regions) may be providedto a third party that generates the mask or write pattern 114-2. Notethat the hotspots 120 may be provided to the third party in a file thatis compatible with a GDS format, such as GDSII.

In some embodiments, different techniques are used to determine the maskor write pattern 114-1 and to determine the modified regions. Forexample, during or after the determining of the mask or write pattern114-1 using pattern generator 112, OPC may be applied to the mask orwrite pattern 114-1. Note that OPC may be applied to the regionscorresponding to the hotspots 120. Moreover, in some embodiments thedetermination of the mask or write pattern 114-2 involves an inverselithographic calculation that includes a projection of the targetpattern 110 at an image plane in the lithographic or photolithographicprocess to an object plane in the lithographic or photolithographicprocess.

Note that in some embodiments verification performed by either or bothof the verifiers 116 is image-based. Moreover, in some embodiments thedetermination of the mask or write pattern 114-2 using thehotspot-repair generator 122 involves image-based and/or pixel-basedcorrection.

While block diagram 100 describes an embodiment in which hotspots 120are extracted prior to being repaired, in other embodiments theseregions are identified and a single file that includes the hotspots 120as well as the remainder of the mask or write pattern 114-1 istransmitted (for example, to the third party) for repair. Processing asingle file may eliminate or reduce the challenges associated withdealing with multiple files for different hotspots 120 and the largenumber of variations in how the hotspots 120 or regions in these filesmay overlap. For example, it may be difficult to break a given hotspotinto pieces because of error propagation (which may not be repairable).

This technique is shown in FIG. 1B, which presents a block diagram 130illustrating an embodiment of determining a mask pattern and/or a writepattern. In particular, hotspot-repair generator 122 may receive thesingle file and may modify the regions or hotspots 120 (FIG. 1A) toproduce the mask or write pattern 114-2. Note that this single file mayinclude the mask or write pattern 114-1 as well as additionalinformation associated with regions or hotspots 120 (FIG. 1A). Moreover,in some embodiments this information is included in single-layer in thefile and/or the file may be compatible with a GDS format, such as aGDSII format.

Note that in some embodiments, block diagram 100 (FIG. 1A) and/or blockdiagram 130 there may be fewer or additional components. For example, atleast a subset of the mask or write pattern 114-1 and/or the hotspots120 (FIG. 1A) may be processed by a group of processors in parallel. Agiven subset or work unit may include or may be within optical proximityof one of the hotspots 120 (FIG. 1A). Moreover, two or more componentsmay be combined into a single component, and/or a position of one ormore components may be changed.

We now describe embodiments of identifying and/or extracting regions orhotspots in a mask pattern and/or a write pattern. In the discussionthat follows, a mask pattern is used as an illustrative example. FIG. 2Apresents a block diagram illustrating an embodiment 200 of a maskpattern 210. In this mask pattern, several hotspots 212 have beenidentified. In an example embodiment, the hotspots 212 include less than5% of a 4 cm² mask or write pattern.

Note that these hotspots correspond to locations (such as seedlocations) associated with violations of the pre-determined rules.Moreover, halos or regions 214 are determined or defined, each of whichsurrounds one of the hotspots 212. As discussed below, the hotspots 212and regions 214 may be used to extract at least a portion of the maskpattern 210 for processing using the hotspot-repair generator 122 (FIGS.1A and 1B).

In some embodiments, the hotspots 212 and/or the regions 214 aredetermined or defined based on one or more geometric relationships forshapes surrounding the locations. For example, the one or more geometricrelationships may include overlapping shapes or adjacent shapes, such aspolygons, that contact each other along an edge. Moreover, in someembodiments the hotspots 212 and/or the regions 214 are determined ordefined by aggregating the shapes.

In an example embodiment, areas associated with the hotspots 212 and theregions 214 are extracted from the mask pattern 210 to produce a sparsefile, which may be compatible with a GDS format. This sparse file may beprocessed as described previously with reference to FIG. 1A.

In some embodiments, the sparse file includes portions (such as hotspots212 and regions 214) of the target layer and/or portions of referencelayers (which include aspects of the design other than the targetpattern) in the mask pattern 210. Note that there may be portions ofbetween 6-32 reference layers in the sparse file. In some embodiments,the sparse file also includes portions of an OPC layer which includesinformation associated with OPC correction.

After processing by the hotspot-repair generator 122 (FIGS. 1A and 1B),an output file may be produced that includes modified areas or regionscorresponding to the hotspots 212 and/or the regions 214. These regions242 are shown in FIG. 2B, which presents a block diagram illustrating anembodiment 230 of a mask pattern 240. As noted previously, the regions240 are expected to comply with the pre-determined rules.

In other embodiments, additional regions each of which surrounds one ofthe hotspots 212 (FIG. 2A) are defined for use during hotspotprocessing. These regions 262 are shown in FIG. 2C, which presents ablock diagram illustrating an embodiment 250 of a mask pattern 260. Inparticular, in some embodiments, during the determination of themodified regions, the hotspots 212 may be modified, and the regions 214and 262 are unmodified. However, in some embodiments at least someregions, such as region 214-3 which is sandwiched between hotspot 212-3and region 262-3, is modified.

In some embodiments, during the determining of the modified regions, amerit or cost function (which is described below in the discussion of aninverse calculation) is determined using hotspots 212 and/or the regions214.

In some embodiments, a size of the regions 214 is 2 times an interactionrange associated with a lithographic or a photolithographic process.Note that the interaction range is a function of a wavelength in thisprocess divided by a numerical aperture associated with an optical path.Moreover, a size of the regions 262 may be 0.2 times the interactionrange.

Note that in some embodiments, an optical characteristic of the regions214 is approximately unchanged or is improved during the determining ofthe modified regions. For example, the optical characteristic mayinclude: the intensity, the contrast, the depth of focus, and/or a maskerror enhancement factor.

In an example embodiment, the single file includes first markerscorresponding to the hotspots 212, the regions 214, and the logical ANDof the hotspots 212 and the regions 214. These markers may be includedin a single layer in the file. Moreover, the first markers may be usedto extract the portion of the target layer, the reference layer(s),and/or the OPC layer associated with the mask or write pattern 114-1(FIGS. 1A and 1B).

In addition, the merge 124 component (FIG. 1A) may use additionalmarkers M1 and M2, where M1 includes the OPC layer minus the firstmarkers and M2 includes the logical AND of the regions 240 (FIG. 213)and the first markers. In particular, the merge 124 component (FIG. 1A)may use the logical OR of M1 and M2.

While files that are compatible with a GDS format, such as a GDSII orOASIS format, have been described previously, in some embodiments one ormore files used during the determination of the mask or write patterns114 (FIGS. 1A and 1B) and/or the hotspots 120 (FIG. 1A) may be flat orhierarchical. Consequently, conversions from one file format to another(and back again) may be performed. For example, a file compatible with aGDSII format may be converted to a pixel-based representation or aformat having fewer levels than the original format. Acceptable fileformats may include those with a pixel-based format (i.e., spatiallydiscrete) data patterns (such as bitmap and/or grayscale images), binarypatterns, and/or continuous-tone patterns.

Note that in some embodiments, mask pattern 210 (FIG. 2A), mask pattern240 (FIG. 2B), and/or mask pattern 250 there may be fewer or additionalcomponents. Moreover, two or more components may be combined into asingle component, and/or a position of one or more components may bechanged.

We now describe embodiments of processes for generating write patternsand/or mask patterns. FIG. 3 presents a flowchart illustrating anembodiment of a process 300 for generating the mask pattern and/or thewrite pattern, which may be performed by a computer system. Duringoperation, this computer system optionally receives a first mask orwrite pattern (310). Next, the computer system optionally analyzes thefirst mask or write pattern using pre-determined rules to identify firstregions that violate the pre-determined rules and to extract at least aportion of the first mask or write pattern (312). Note that thepre-determined rules may be associated with a photo-mask or alithographic process, such as a photolithographic process or amaskless-lithography process. Alternatively, the computer systemoptionally receives at least a portion of the first mask or writepattern including the first regions (314).

Then, the computer system determines a second mask or write patternbased on at least the portion of the first mask or write pattern, wherethe second mask or write pattern includes second regions that areestimated to comply with the pre-determined rules (316). Note that thesecond regions correspond to the first regions, and the second mask orwrite pattern is determined using a different technique than that usedto determine the first mask or write pattern.

In some embodiments, the computer system optionally analyzes the secondmask or write pattern using the pre-determined rules to confirm that thesecond regions comply with the pre-determined rules (318). Moreover, insome embodiments, the computer system optionally merges the second maskor write pattern with a remainder of the first mask or write pattern,where the remainder of the first mask or write pattern excludes thefirst regions (320).

FIG. 4 presents a flowchart illustrating an embodiment of a process 400for generating the mask pattern and/or the write pattern, which may beperformed by a computer system. During operation, this computer systemreceives a file including a first mask or write pattern and additionalinformation associated with regions in the first mask or write patternthat violate pre-determined rules (410). Next, the computer systemdetermines a second mask or write pattern based on the first mask orwrite pattern, where the second mask or write pattern is estimated tocomply with the pre-determined rules (412). Moreover, in someembodiments, the computer system optionally analyzes the second mask orwrite pattern using the predetermined rules to confirm that the secondmask or write pattern complies with the pre-determined rules (414).

Note that in some embodiments of the process 300 (FIG. 3) and/or 400there may be additional or fewer operations. Moreover, the order of theoperations may be changed and/or two or more operations may be combinedinto a single operation.

We now describe embodiments of an inverse calculation (such as aninverse lithography calculation) that may be used to determine orgenerate one or more write patterns and/or mask patterns. This inversecalculation may be based on minimization of an error function (which isalso sometimes referred to as a cost function or a Hamiltonianfunction). During each iteration of the calculation, an error functionmay be a function of the difference between the estimated wafer patternthat results when the aerial image or printed pattern associated withthe write pattern or the mask pattern is projected through an opticalpath in an optical system during a lithographic process. In someembodiments, multiple images may be used, such as those corresponding toa range of process conditions or multiple exposures.

A forward calculation may also be used when determining the costfunction. In the discussion that follows, coherent illumination by anillumination pattern associated with the source in the lithographicprocess is assumed. Furthermore, the electric field falling on thephoto-mask (which is associated with the mask pattern) or provided by awrite device (based on the write pattern) is approximately constant.Using photo-lithography as an example, note that some regions of thephoto-mask (such as those corresponding to ‘0s’ in a binary maskpattern) reflect the light to the semiconductor wafer at the image planeof the optical system, while other regions (such as those correspondingto ‘1s’ in the binary mask pattern) do not reflect the light to thesemiconductor wafer. It follows that a scalar electric field E afterreflection off of the photo-mask (which is henceforth referred to as animage pattern), may be expressed as

${{E\left( \overset{\_}{r} \right)} = \begin{Bmatrix}0 \\1\end{Bmatrix}},$

where {right arrow over (r)}=(x,y) is a point on the (x,y) plane. Moregenerally, the electric field may include grayscale values in the range{−1, 1} or arbitrary complex numbers that correspond to the features ofthe photo-mask. Thus, chrome-on-glass photo-masks and various types ofphoto-masks other than chrome on glass, such as attenuated phaseshifting, strong phase shifting, other materials, etc., are within thescope of the present invention.) Note that this representation of theimage pattern may be re-expressed using a function φ (referred to as alevel-set function) having positive regions that indicate lightreflected to the semiconductor wafer and negative regions that indicatean absence of light reflected to the semiconductor wafer. (Moregenerally, the level-set function may have two or more levels and agiven mask pattern or write pattern may be represented by one or morelevel-set functions.) Furthermore, the level-set function may equal zeroat the boundaries or contours of features of the photo-mask. Therefore,the electric field E associated with the photo-mask (and thus, the maskpattern) may be re-expressed as a function of this level-set function,i.e.,

E({right arrow over (r)})=ĥ(Φ(x,y)),

where ĥ is a transmission or reflection function

${\hat{h}(x)} = {\begin{Bmatrix}1 & {x \geq 0} \\0 & {x < 0}\end{Bmatrix}.}$

Since an ideal diffraction limited lens acts as a low-pass filter, thismay be used as an approximation to the actual (almost but not quiteperfect) optics in the optical path in the optical system.Mathematically, the action of a lens may be expressed as

A({right arrow over (r)})=f ⁻¹(Ĉ(f(E({right arrow over (r)}))))

where A({right arrow over (r)}) indicates the electric fielddistribution on the wafer, f indicates the Fourier transform, f⁻¹indicates the inverse Fourier transform, and Ĉ indicates the pupilcutoff function, which is zero for frequencies larger than a thresholddetermined by the numerical aperture of the lens, and one otherwise.Thus, the pupil function is

${{\overset{\Cap}{C}\left( {k_{x},k_{y}} \right)} = {{\hat{h}\left( {k_{\max}^{2} - \left\lbrack {k_{x}^{2} + k_{y}^{2}} \right\rbrack} \right)} = \begin{Bmatrix}0 & {{k_{x}^{2} + k_{y}^{2}} \geq k_{\max}^{2}} \\1 & {{k_{x}^{2} + k_{y}^{2}} < k_{\max}^{2}}\end{Bmatrix}}},$

wherein k_(x), k_(y) and k_(max) represent frequency coordinates inFourier space. Therefore, the intensity pattern on the semiconductorwafer is simply the square of the electric field

I({right arrow over (r)})=|A({right arrow over (r)})|².

Combining these two equations, we find

F(φ(x,y))=(|f ⁻¹(Ĉ(f(ĥ(Φ(x,y)))))|²).

This is a self-contained formula for the intensity pattern at thesemiconductor wafer.

Note that this is just one embodiment of the forward calculation thatcan be used within the scope of this invention, chosen by way of exampledue to its relative simplicity. More sophisticated forward models alsofall within the scope of the present invention. Such models may takeinto account, by way of example but not limitation: various illuminationconditions (e.g., off-axis, incoherent), the actual electromagnetics ofthe light field interacting with the photo-mask, the polarization of thelight field, the actual properties of the lens (such as aberrations),and/or the vector nature of the electromagnetic field as it propagatesthrough the optical path in the optical system during the lithographicprocess.

In some embodiments, during each iteration of the inverse calculationthe level-set function corresponding to the mask pattern and/or thewrite pattern is updated according to

φ_(i+1)=φ_(i)+Δφ=φ_(i) +Δt·∇(H),

where Φ_(i+1) is an updated version of the level-set function, Φ_(i) isthe current version of the level-set function, Δt is a step size in thecalculation and ∇(H) is a gradient or a derivative of the errorfunction. Note that ∇(H) is

$\left. \frac{\delta \; H}{\delta\varphi} \right|_{\varphi_{i}},$

i.e., it is the Frechet derivative of the cost function H. Furthermore,in some embodiments ∇(H) is the direction of steepest descent forminimizing or optimizing H by changing Φ. Furthermore, in someembodiments a 1^(st) order and/or a 3^(rd) order Runge-Kutta method isused when updating Φ_(i). In other embodiments, a Conjugate Gradienttechnique, a Levenberg-Marquardt technique, a Quasi-Newton technique,and/or a Simplex technique may be used.

At least some aspects of Simulated Annealing may be utilized in someembodiments of the inverse calculation. In particular, the errorfunction H may be allowed to increase during some iterations as thecalculation evolves. In this way, the global minimum in themulti-dimensional space may be determined. Note that the size of thismulti-dimensional space is proportional to a number of quantizationlevels to the power of the number of pixels in the mask pattern or thewrite pattern. In an example embodiment, these patterns or images haveat least 1 million pixels (for example, 1024×1024).

In some embodiments, features are added to the mask pattern or the writepattern during the inverse calculation. For example, features associatedwith sub-resolution assist features (SRAFs) may be added at one or morelocations (such as at a maximum and/or a minimum in the gradient of thecost function). These features may help improve the mask pattern or thewrite pattern and/or may help accelerate convergence of the inversecalculation.

In some embodiments, the inverse calculation is divided into a series ofoverlapping work units, at least some of which are processedindependently and/or concurrently. These work units may be based onfeatures or structures (for example, repetitive structures) in the maskpattern or the write pattern.

In some embodiments, the inverse calculation is run for 100, 1000 or10,000 iterations at which point the optimal solution has beendetermined. In other embodiments, the calculation is stopped based onconvergence criteria, such as: oscillatory behavior; a relative and/orabsolute difference between the target pattern and an estimate of theprinted pattern or developed wafer pattern; the latest change to theerror function H; and/or the history of changes to the error function H.For example, the relative difference may be less than 1% and/or theabsolute difference may be 10 nm for a critical dimension of 100 nm.

We now describe embodiments of computer systems that perform thedetermining or generating of the mask patterns and/or write patterns.FIG. 5 presents a block diagram illustrating an embodiment of a computersystem to generate the mask pattern and/or the write pattern. Computersystem 500 includes multiple processors 510 or processor cores, acommunication interface 512, a user interface 514, and one or moresignal lines 522 coupling these components together. Note that theprocessing units 510 support parallel processing and/or multi-threadedoperation, the communication interface 512 may have a persistentcommunication connection, and the one or more signal lines 522 mayconstitute a communication bus. Moreover, the user interface 514 mayinclude a display 516, a keyboard 518, and/or a pointer 520, such as amouse.

Memory 524 in the computer system 500 may include volatile memory and/ornon-volatile memory. More specifically, memory 524 may include ROM, RAM,EPROM, EEPROM, FLASH, one or more smart cards, one or more magnetic discstorage devices, and/or one or more optical storage devices. Memory 524may store an operating system 526 that includes procedures (or a set ofinstructions) for handling various basic system services for performinghardware dependent tasks. The memory 524 may also store communicationsprocedures (or a set of instructions) in a communication module 528. Thecommunication procedures may be used for communicating with one or morecomputers, devices and/or servers, including computers, devices and/orservers that are remotely located with respect to the computer system500.

Memory 524 may also include one or more program modules (or a set ofinstructions), including: first computation module 530 (or a set ofinstructions), second computation module 532 (or a set of instructions),verification module 534 (or a set of instructions), extraction module536 (or a set of instructions), and/or merge module 538 (or a set ofinstructions). First computation module 530 may determine one or moremask and/or write patterns 550 (such as pattern A 552-1, pattern B552-2, and/or pattern N 552-N). These patterns may be analyzed usingverification module 534 to identify regions that violate pre-determinedrules, such as manufacturing criteria 540 and/or feature rules 542.

Next, extraction module 536 may extract at least these regions andgenerate portions of one or more mask and/or write patterns 546 (such ashotspot A 548-1, hotspot B 548-2, and/or hotspot M 548-M), as well asone or more optional remainders 544 (which include the remainder of theone or more mask or write patterns, i.e., which exclude the regions).Then, second computation module 532 may determine additional regionsusing these portions. These additional regions, which correspond to theregions, may comply with the pre-determined rules. Alternatively, secondcomputation module 532 may determine additional mask or write patterns(not shown) from a file that includes one or more of the mask or writepatterns 550 as well as information associated with the regions.

In some embodiments, verification module 534 is used to confirm that theadditional regions and/or the additional mask or write patterns complywith the pre-determined rules. Moreover, in some embodiments mergemodule 538 combines the additional regions with the optionalremainder(s) 544 to generate the additional mask or write patterns (notshown).

Instructions in the various modules in the memory 524 may be implementedin a high-level procedural language, an object-oriented programminglanguage, and/or in an assembly or machine language. The programminglanguage may be compiled or interpreted, i.e., configurable orconfigured to be executed by the processing units 510.

Although the computer system 500 is illustrated as having a number ofdiscrete components, FIG. 5 is intended to be a functional descriptionof the various features that may be present in the computer system 500rather than as a structural schematic of the embodiments describedherein. In practice, and as recognized by those of ordinary skill in theart, the functions of the computer system 500 may be distributed over alarge number of servers or computers, with various groups of the serversor computers performing particular subsets of the functions. In someembodiments, some or all of the functionality of the computer system 500may be implemented in one or more ASICs and/or one or more digitalsignal processors DSPs.

Note that computer system 500 may include fewer components or additionalcomponents. Moreover, two or more components may be combined into asingle component, and/or a position of one or more components may bechanged. In some embodiments the functionality of computer system 500may be implemented more in hardware and less in software, or less inhardware and more in software, as is known in the art.

We now describe embodiments of a data structure that may be used in thecomputer system 500. FIG. 6 presents a block diagram illustrating anembodiment of a data structure 600. This data structure may includeinformation associated with one or more versions of mask patterns and/orwrite patterns (such as patterns 610). In particular, a given pattern(such as pattern 610-1) may include pixels 612 and associated values614.

FIG. 7 presents a block diagram illustrating an embodiment of a datastructure 700. This data structure may include information associatedwith one or more versions of mask patterns and/or write patterns (suchas patterns 710). In particular, a given pattern (such as pattern 710-1)may include: pixels 712, associated values 714, and/or additionalinformation 716 associated with regions in the pattern 710-1 thatviolate pre-determined rules (such as manufacturing criteria 540 and/orfeature rules 542 in FIG. 5).

Note that in some embodiments of the data structure 600 (FIG. 6) and/orthe data structure 700 there may be fewer or additional components.Moreover, two or more components may be combined into a singlecomponent, and/or a position of one or more components may be changed.

The foregoing descriptions of embodiments of the present invention havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A computer-implemented method for determining a mask pattern to beused on a photo-mask in a lithography process, comprising: receiving atleast a portion of a first mask pattern including first regions thatviolate pre-determined rules associated with the photo-mask; anddetermining a second mask pattern based on at least the portion of thefirst mask pattern, wherein the second mask pattern includes secondregions that are estimated to comply with the pre-determined rules,wherein the second regions correspond to the first regions, and whereinthe second mask pattern is determined using a different technique thanthat used to determine the first mask pattern.
 2. The method of claim 1,further comprising: analyzing the first mask pattern using thepre-determined rules to identify the first regions; and extracting atleast the portion of the first mask pattern.
 3. The method of claim 2,wherein the analysis of the first mask pattern involves verification. 4.The method of claim 3, wherein the verification is image-based.
 5. Themethod of claim 2, wherein identification of the first regions involvesidentifying locations associated with violations of the pre-determinedrules, and wherein the extraction of at least the portion of the firstmask pattern involves calculating the first regions based on one or moregeometric relationships for shapes surrounding the locations.
 6. Themethod of claim 5, wherein the one or more geometric relationshipsinclude overlapping shapes or adjacent shapes that contact each otheralong an edge.
 7. The method of claim 6, wherein the shapes includepolygons.
 8. The method of claim 5, wherein the extraction of at leastthe portion of the first mask pattern involves aggregating thecalculated first regions.
 9. The method of claim 1, wherein receivinginvolves accessing at least the portion of the first mask pattern in acomputer-readable memory.
 10. The method of claim 1, further comprisingapplying optical proximity correction to at least the portion of thefirst mask pattern prior to the determination of the second maskpattern.
 11. The method of claim 1, further comprising merging thesecond mask pattern with a remainder of the first mask pattern, whereinthe remainder of the first mask pattern excludes the first regions. 12.The method of claim 1, wherein the first regions include hotspots. 13.The method of claim 1, wherein the pre-determined rules includemanufacturing rules associated with the photo-mask.
 14. The method ofclaim 1, wherein at least the portion of the first mask pattern furtherincludes third regions: wherein a given third region surrounds a givenfirst region; and wherein the third regions are unchanged during thedetermination of the second mask pattern.
 15. The method of claim 1,wherein at least the portion of the first mask pattern further includesthird regions: wherein a given third region surrounds a given firstregion; wherein a first portion of the third regions are unchangedduring the determination of the second mask pattern; wherein a secondportion of the third regions are changed during the determination of thesecond mask pattern; and wherein the second portion of the third regionsis a transition region between the second regions and the first portionof the second regions.
 16. The method of claim 15, wherein an opticalcharacteristic of the second portion of the third regions isapproximately unchanged or is improved relative to the opticalcharacteristic of the second portion of the third regions in at leastthe portion of the first mask pattern.
 17. The method of claim 16,wherein the optical characteristic includes intensity, contrast, depthof focus, or a mask error enhancement factor.
 18. The method of claim 1,wherein the first mask pattern includes multiple subsets that are to beprocessed by a group of processors, and wherein the portion of the firstmask pattern includes those multiple subsets that are proximate to thefirst regions.
 19. The method of claim 1, further comprising analyzingthe second mask pattern using the pre-determined rules to confirm thatthe second regions comply with the pre-determined rules.
 20. The methodof claim 19, wherein the analysis of the second mask pattern involvesverification.
 21. The method of claim 1, wherein in the determination ofthe second mask pattern involves an inverse lithographic calculationthat includes a projection of a target pattern at an image plane in thephotolithographic process to an object plane in the photolithographicprocess.
 22. The method of claim 1, wherein the determination of thesecond mask pattern involves image-based or pixel-based correction. 23.The method of claim 1, wherein at least the portion of the first maskpattern is included in a file that is compatible with a GDSII format.24. A computer-program product for use in conjunction with a computersystem, the computer-program product comprising a computer-readablestorage medium and a computer-program mechanism embedded therein fordetermining a mask pattern to be used on a photo-mask in a lithographyprocess, the computer-program mechanism including. instructions forreceiving at least a portion of a first mask pattern including firstregions that violate pre-determined rules associated with thephoto-mask; and instructions for determining a second mask pattern basedon at least the portion of the first mask pattern, wherein the secondmask pattern includes second regions that are estimated to comply withthe pre-determined rules, wherein the second regions correspond to thefirst regions, and wherein the second mask pattern is determined using adifferent technique than that used to determine the first mask pattern.25. A computer system to determine a mask pattern to be used on aphoto-mask in a lithography process, comprising: at least one processor;at least one memory; and at least one program module, the program modulestored in the memory and configured to be executed by the processor, atleast the program module including: instructions for receiving at leasta portion of a first mask pattern including first regions that violatepre-determined rules associated with the photo-mask; and instructionsfor determining a second mask pattern based on at least the portion ofthe first mask pattern, wherein the second mask pattern includes secondregions that are estimated to comply with the pre-determined rules,wherein the second regions correspond to the first regions, and whereinthe second mask pattern is determined using a different technique thanthat used to determine the first mask pattern.